In the art of preparing S-chloromethyl derivatives of O-alkyl substituted phosphodithioic acids, the accepted practice has involved reacting a suitable salt of such an acid with bromochloromethane under conditions favoring conversion of the salt forming moiety to a bromide salt, as illustrated by U.S. Pat. Nos. 3,020,304 (Scherer et al.) and 3,896,219 (Pianka). A related process incorporating particular, specialized conditions under which a more readily available reactant, namely dichloromethane, can be used effectively as the S-chloromethylating agent forms the subject of copending U.S. Application Ser. No. 053,066 filed on June 28, 1979, by W. W. Brand and having a common assignee with the present application.
The present invention is directed to entirely different methods for preparing S-chloromethyl derivatives of O-alkyl substituted phosphodithioic acids involving the reactions of their S-hydroxymethyl analogs with suitable acid chlorides. The choice of suitable acid chloride reactants is rather wide and includes such common inorganic species as phosphorus pentachloride and phosphorus trichloride. Indeed, due to the ready availability and high reactivity of the acid chloride reagents employed, the novel process of the present invention provides several important advantages in the manufacture of the desired derivatives. For example, lower temperature operations are rendered more practical, including efficient conversions at ordinary room temperatures (or even below). In fact, in many cases, the selectivity of the reaction with regard to yield of the desired derivative is close to optimum at ordinary room temperatures. Accordingly, the subject processes are very energy efficient and economical to operate, especially since a large excess of main reactant is not needed and product recovery is straight forward.